It is highly desirable to produce copolymers of maleic anhydride and alkyl vinyl ethers with substantially reduced benzene contents and preferably which are substantially benzene free.
Copolymers of maleic anhydride and alkyl vinyl ether have a variety of important commercial uses, in the medical, cosmetic, pharmaceutical, food and other industries. One important property of the copolymer for these applications is its molecular weight, as commonly measured by its specific viscosity of a 1% (wt./vol.) solution in methyl ethyl ketone at 25.degree. C. For example, U.S. Pat. No. 3,003,988, teaches that a useful denture stabilizing composition is obtained from such copolymers. For many applications, the benzene content of the copolymer is an important property.
Benzene is a recognized human carcinogen. Hence, benzene specifications have become extremely important for personal care products, and benzene contents are continually being reduced. In 1990, the New York State Consumer Protection Bureau, in conjunction with the FDA, announced the results of a study of the benzene content of denture adhesives on the market, and asked manufacturers with products exceeding 10 parts per billion (ppb) of benzene to remove them from the market. This level represents the current limit of detectability of benzene in denture adhesives, and future testing may evolve more sensitive detection methods and more stringent controls on benzene content.
Because benzene is a recognized carcinogen, it thus becomes necessary to manufacture personal care products with reduced benzene levels. One such specialty product in wide use in personal care products is the copolymer of maleic anhydride and alkyl vinyl ether. It is used either directly or as derivative products of the copolymer in denture adhesives, detergent bars, hair sprays, toothpastes, and the like.
Production of interpolymers of methyl vinyl ether and maleic anhydride are generally conducted by dissolving the reactants, along with a free radical initiator, in a material which is a solvent for the reaction mix, which has no undesirable reactions with any component of the reaction mix, and which does not interfere with the free radical polymerization reaction which builds molecular weight. This latter criterion is extremely difficult to meet and the choice of practical solvents is thus restricted. In addition, the material chosen as a solvent for the reaction mix is preferably a nonsolvent for the polymer product formed. The polymer thus becomes a slurry of finely divided particles as formed during the reaction, and can be separated easily from the solvent.
Maleic anhydride-alkyl vinyl ether copolymers have historically been produced in a benzene solvent, as disclosed in U.S. Pat. Nos. 2,782,182 and 3,532,771. Product produced in a benzene solvent is inherently of stable shelf life in terms of viscosity, and exhibits no viscosity degradation when exposed to temperatures of 30 to 90.degree. C. The usual product sold in commerce from this reaction system contains up to 2% benzene, commonly 1.5% benzene, on a weight percent of copolymer. Even extensive drying, at such extreme conditions as to be uneconomic commercially, can only reduce the benzene content of the copolymer to about 0.5% to 1.0% by weight of copolymer. This level far exceeds the acceptable concentration of benzene.
U.S. Pat. No. 4,900,809 discloses a method of removal of benzene from the copolymer, consisting of extraction of the benzene from the copolymer with aromatic solvents, such as toluene. This approach requires high reflux ratios, is costly, it contaminates the copolymer with toluene (a component of gasoline), and still does not reduce benzene levels below about 1 ppm (1000 ppb).
It would be most desirable to identify other solvents than benzene for the conduct of the polymerization of maleic anhydride and alkyl vinyl ether. This would overcome the many disadvantages of extraction of the benzene from the copolymer after it is formed in a benzene solvent. Because of the impact on high molecular weight copolymer production, the identification of a suitable solvent is quite difficult. The use of a methylene chloride solvent for this purpose is disclosed in U.S. Pat. No. 3,632,361. Unfortunately, methylene chloride has also been identified to be a carcinogen.
The use of acetone or ethyl acetate solvents is disclosed in U.S. Pat. No. 4,948,848. This procedure requires a long time period for continuous feed of maleic anhydride dissolved in the solvent, presumably to permit the desired growth in molecular weight. Further, since acetone and ethyl acetate are solvents for the copolymer itself, care must be taken to prevent the dissolution of the copolymer in the solvent by control of the post-reaction operations. In the examples of this patent, toluene was added to the reaction product to maintain the reaction product in a pumpable form. Toluene, of course, exhibits a boiling point very near that of benzene, and is thus not available in a form free of benzene contamination. This patent also discloses that use of high ratios of methyl vinyl ether to maleic anhydride will product high molecular weight copolymer, which is very desirable for many applications.
U.S. Pat. No. 5,047,490 discloses methods of conducting the polymerization only in the presence of alkyl vinyl ether, thereby eliminating the use of any extraneous solvent and thus providing the prospect of extremely high purity copolymers of alkyl vinyl ether and maleic anhydride. They also teach the use of a high ratio of methyl vinyl ether to maleic anhydride, from about 3 to about 20 moles of methyl vinyl ether per mole of maleic anhydride.
Even when following the teachings of U.S. Pat. No. 5,047,490, it has surprisingly been found that the copolymer contains substantial amounts of benzene, perhaps as much as 600 ppb. In these tests, lauryl peroxide was used as a free radical initiator and is expected to be free of benzene by one skilled in the art due to its manufacturing method. Maleic anhydride is now commonly produced by the oxidation of 4 carbon hydrocarbon molecules, and is expected to be free of benzene by one skilled in the art due to its manufacturing method. Alkyl vinyl ethers, and specifically methyl vinyl ether, are commonly produced either by reactions of an alcohol with acetylene or by reaction of alcohol with vinyl acetate. Because of the manufacturing steps used to effectively cleanse the acetylene from benzene and the absence of benzene in vinyl acetate and alcohol, one skilled in the art would expect the alkyl vinyl ether to be substantially free of benzene due to its manufacturing method.
Further, since excess methyl vinyl ether or other solvents used in the copolymerization reaction are often evaporated from the copolymer to produce the dry product, one skilled in the art would expect substantially all of any contaminating benzene to also be distilled over and thence removed from the dried copolymer product. For example, U.S. Pat. No. 5,047,490 uses 3 to 20 moles of methyl vinyl ether per mole of maleic anhydride. The vapor-liquid equilibrium calculations for evaporating a methyl vinyl ether present in an amount of 3 to 20 parts methyl vinyl ether for 1 part copolymer, and which also contains 600 parts of benzene for 1 billion parts of copolymer, would lead one skilled in the art to expect substantially all of the benzene to be contained in the methyl vinyl ether distillate and de minimis benzene to be contained in the dried copolymer. Mass spectrometry-gas chromatographic analyses of the distillate product do not show the presence of benzene and yet the dried copolymer can contain 600 ppb of benzene.